The present invention relates generally to data transmission in mobile communication systems and more specifically to methods for sharing a control channel for carrier aggregation.
As used herein, the terms “user agent” and “UA” can refer to wireless devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices or other User Equipment (“UE”) that have telecommunications capabilities. In some embodiments, a UA may refer to a mobile, wireless device. The term “UA” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems/equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the term “access device” will refer to any component, such as a traditional base station or an LTE or LTE-A access device (including eNBs), that can provide a UA with access to other components in a telecommunications system.
In mobile communication systems such as E-UTRAN, an access device provides radio access to one or more UAs. The access device comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UAs communicating with the access device. The functions of the scheduler include, among others, dividing the available air interface capacity between UAs, deciding the transport channel to be used for each UA's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UAs through a scheduling channel.
Several different data control information (DCI) message formats are used to communicate resource assignments to UAs including, among others, a DCI format 0 for specifying uplink resources, DCI formats 1, 1A, 1B, 1C, 1D, 2 and 2A for specifying downlink resources, and DCI formats 3 and 3A for specifying power control information. Uplink specifying DCI format 0 includes several DCI fields, each of which includes information for specifying a different aspect of allocated uplink resources. Exemplary DCI format 0 DCI fields include a transmit power control (TPC) field, a cyclic shift for demodulation reference signal (DM-RS) field, a modulation and coding scheme (MCS) and redundancy version field, a New Data Indicator (NDI) field, a resource block assignment field and a hopping flag field. The downlink specifying DCI formats 1, 1A, 2 and 2A each include several DCI fields that include information for specifying different aspects of allocated downlink resources. Exemplary DCI format 1, 1A, 2 and 2A DCI fields include a HARQ process number field, an MCS field, a New Data Indicator (NDI) field, a resource block assignment field and a redundancy version field. Each of the DCI formats 0, 1, 2, 1A and 2A includes additional fields for specifying allocated resources. Other downlink formats 1B, 1C and 1D include similar information. The access device selects one of the downlink DCI formats for allocating resources to a UA as a function of several factors including UA and access device capabilities, the amount of data a UA has to transmit, the communication (channel) condition, the transmission mode to be used, the amount of communication traffic within a cell, etc.
DCI messages are synchronized with sub-frames so that they can be associated therewith implicitly as opposed to explicitly, which reduces control overhead requirements. For example, in LTE frequency division duplex (FDD) systems, a DCI message for uplink resource is associated with an uplink sub-frame four milliseconds later so that, for example, when a DCI message is received at a first time, the UA is programmed to use the resource grant indicated therein to transmit a data packet in the sub-frame four milliseconds after the first time. Similarly, a DCI message for downlink resource is associated with a simultaneously transmitted downlink sub-frame. For example, when a DCI message is received at a first time, the UA is programmed to use the resource grant indicated therein to decode a data packet in a simultaneously received traffic data sub-frame.
During operation, LTE networks use a shared Physical Downlink Control CHannel (PDCCH) to distribute DCI messages amongst UAs. The DCI messages for each UA as well as other shared control information are separately encoded. In LTE, PDCCHs are transmitted in the first few OFDM symbols over the whole system bandwidth, which can be called a PDCCH region. The PDCCH region includes a plurality of control channel elements (CCEs) that are used to transmit DCI messages from an access device to UAs. An access device selects one or an aggregation of CCEs to be used to transmit a DCI message to a UA, the CCE subset selected to transmit a message depends at least in part on perceived communication conditions between the access device and the UA. For example, where a high quality communication link is known to exist between an access device and a UA, the access device may transmit data to the UA via a single one of the CCEs and, where the link is low quality, the access device may transmit data to the UA via a subset of two, four or even eight CCEs, where the additional CCEs facilitate a more robust transmission of an associated DCI message. The access device may select CCE subsets for DCI message transmission based on many other criteria.
Because a UA does not know exactly which CCE subset or subsets are used by an access device to transmit DCI messages to the UA, in existing LTE networks, the UA is programmed to attempt to decode many different CCE subset candidates when searching for a DCI message. For instance, a UA may be programmed to search a plurality of single CCEs for DCI messages and a plurality of two CCE subsets, four CCE subsets and eight CCE subsets to locate a DCI message. To reduce the possible CCE subsets that need to be searched, access devices and UAs have been programmed so that each access device only uses specific CCE subsets to transmit DCI messages to a specific UA corresponding to a specific data traffic sub-frame and so that the UA knows which CCE subsets to search. For instance, in current LTE networks, for each data traffic sub-frame, a UA searches six single CCEs, six 2-CCE subsets, two 4-CCE subsets and two 8-CCE subsets for DCI messages for a total of sixteen CCE subsets. The sixteen CCE subsets are a function of a specific Radio Network Temporary Identifier (RNTI) assigned to a UA 10 and vary from one sub-frame to the next. This search space that is specific to a given UA is referred to hereinafter as “UA specific search space”.
In many cases it is desirable for an access device to transmit a large amount of data to a UA or for a UA to transmit large amounts of data to an access device in a short amount of time. For instance, a series of pictures may have to be transmitted to an access device over a short amount of time. As another instance, a UA may run several applications that all have to receive data packets from an access device essentially simultaneously so that the combined data transfer is extremely large. One way to increase the rate of data transmission is to use multiple carriers (i.e., multiple frequencies) to communicate between an access device and UAs, as is the case for LTE-A. For example, a system may support five different carriers (i.e. frequencies) and eight HARQ processes so that five separate eight uplink HARQ and five separate eight downlink HARQ transmission streams can be generated in parallel. Communication via multiple carriers is referred to as carrier aggregation.
In the case of carrier aggregation, a control-channel structure is allocated to each carrier for distributing DCI control messages. As a simple way, each carrier can include a separate PDCCH region allowing control channel information to be communicated between the access device and UAs for each carrier independently. This approach, while allowing for control channel information to be distributed for each carrier, requires the allocation of a substantial amount of resources on each carrier. Furthermore, because the level of interference varies amongst carriers, it may be difficult to implement PDCCH regions on all carriers equally. In some cases, for example, the interference levels on a particular carrier may be so substantial as to make it difficult or impossible to implement a PDCCH region on that carrier. Alternatively, the DCI message format for control messages on a first carrier may be modified to provide an additional field for indicating a specific carrier associated with each DCI message. This solution, however, is undesirable as it is currently undesirable to modify DCI formats.